insult meaning someone who is mentally retarded, derived from the word spastic
usually directed at someone after they show a weird trait, do something against a widely accepted moral standard, show themselves to be unskilled or inferior at a particular skill or task, do something that would be associated with a mentally unstable person, or show their stupidity in an obvious way.
"that jonny is a spacca, he keeps twitching"
"stop beating that old lady, you spacca!"
"greg's a spacca, i beat his ass at pool so many times"
"stop pulling my hair you spacca!"
"if you don't know who david hasselhoff is, you're a spacca"